The present invention relates to an extreme ultraviolet light source device and a method for generating extreme ultra violet.
For example, a semiconductor chip may be created by reduction projection of a mask on which a circuit pattern is drawn onto a wafer having a resist applied thereon, and by repeatedly performing processing, such as etching and of thin film formation. The progressive reduction of the scale of semiconductor processing demands the use of radiation of further short wavelength.
Thus, research is being made on a semiconductor exposure technique which uses radiation of extremely short wavelength of 13.5 nm or so and a reduction optics system. This type of technique is termed EUVL (Extreme Ultra Violet Lithography: exposure using extreme ultra violet light). Hereinafter, extreme ultraviolet light will be abbreviated as “EUV light”.
Three types of EUV light sources are known: an LPP (Laser Produced Plasma: plasma produced by a laser) type light source, a DPP (Discharge Produced Plasma) type light source, and an SR (Synchrotron Radiation) type light source. The LPP type light source is a light source which generates a plasma by irradiating laser beam on a target material, and employs EUV radiation emitted from this plasma. The DPP type light source is a light source which employs a plasma generated by an electrical discharge. The SR (synchrotron radiation) is a light source which uses orbital radiation. Of those three types of light sources, the LPP type light source is more likely to obtain high-output EUV radiation as compared to the other two types because the LPP type light source can provide an increased plasma density, and can ensure a larger solid angle over which the radiation is collected.
Since EUV radiation has a very short wavelength and can easily be absorbed by a matter, the EUVL uses a reflection type optical system. Such a reflection type optical system is built by employing a multilayer film in which, for example, molybdenum (Mo) and silicon (Si) are used. Since an Mo/Si multilayer film has a high reflectivity of near 13.5 nm, EUV radiation of a wavelength of 13.5 nm is used in the EUVL.
Since the reflectivity of the multilayer film is around 70%, however, the output gradually decreases as the reflection is repeated. Since the EUV radiation is reflected more than ten times within the exposure device, it is necessary for the EUV light source device to supply high-output EUV radiation to the exposure device. It is therefore expected that the use of an LPP type light source as an EUV light source device will become more popular (see JP-A-2006-80255).
LPP type EUV light source devices use tin (Sn), xenon (Xe), lithium (Li) or the like as a target material, and irradiate laser beam thereon. Particularly, an LPP type light source that uses a combination of tin droplets, which is a liquid metal, and a carbon dioxide (CO2) pulsed laser is promising for this light source because it can reduce the masses of the targets and have a relatively high emission efficiency of EUV radiation as compared with the other LPP type light sources.
To obtain a high EUV radiation emission efficiency, the density of a target needs to be set to about 1017/cm3 to 1018/cm3. The density of solid or liquid tin is however 4×1022 or so which is higher than the optimal density. It is not therefore possible to efficiently obtain EUV radiation through a single irradiation of laser beam. In this respect, there has been proposed a technique of adjusting the density of a tin target by irradiating laser beam on the tin target two times (see the specification of USP-A-2006/0255298 and the pamphlet of WO2003/096764). In this technique, a heating pulsed laser beam is irradiated on a tin target to diffuse the tin target and reduce the density thereof. Then, a main pulsed laser beam is irradiated on the tin target to turn into plasma the target, thereby efficiently generating EUV radiation.
See “Principles of Charged Particle Acceleration written by Stanley Humphries, Jr. (published by John Wiley & Sons, Inc.) too.
According to the related art, a target material is supplied in the form of droplets with a diameter of, for example, several tens of μm. However, only 1/10 of the total mass of the droplets or less actually becomes a plasma which contributes to generation of EUV radiation, while the remainder mass becomes minute particles called debris. It is a problem of the related art that the debris damages an EUV collector mirror, thereby reducing the EUV radiation output.
An EUV collector mirror, which collects EUV radiation radiated from a plasma and supplies the laser beam to the exposure device, is provided in the vicinity of the point of generating the plasma. As the debris which is electrically neutral is diffused to the EUV collector mirror, the life and reflectivity of the EUV collector mirror are reduced. For example, fast debris collides against the top surface of the EUV collector mirror, damaging the EUV collector mirror. Middle speed debris is deposited on the top surface of the EUV collector mirror, lowering the reflectivity of the EUV collector mirror.
When a metal material like tin is used as a target material, therefore, a large quantity of electrically neutral debris is produced, which significantly shortens the lifetime of the EUV collector mirror or the like. Because most of debris is electrically neutral, it is difficult to control the behavior of the debris with electromagnetic force. Accordingly, the related art does not efficiently restrain the diffusion of debris to the EUV collector mirror. When the EUV light source device is operated, therefore, the debris damages the EUV collector mirror, thus making it necessary to frequently perform a work of replacing the EUV collector mirror or the like. As a result, the operation rate of the EUV light source device drops.
Meanwhile, the related art that irradiates laser beam twice can obtain EUV light with a little higher conversion efficiency. This related art is not much different in that a wasteful material which does not contribute to a plasma is supplied into the plasma generation chamber, thereby generating electrically neutral debris.